The present invention relates to an improved deflection amplifier used as a driver for high inductance loads. The present invention finds its application principally as an amplifier for driving deflection coils of cathode ray tubes. The amplifier is also useful in those systems where the signal applied to the inductive load is in the form of a saw-tooth wave.
The present invention, however, is to be distinguished from such systems wherein the driver for the deflection coils or inductive load is a resonance scan system or fly-back system such as is commonly utilized in conventional television sets. The resonance scan system or fly-back scan system of the common TV is a circuit which has become highly efficient since the early days of television. It will be recognized by those skilled in the art as including a deflection system including a capacitor and the yoke of the CRT (deflection coil) plus a power driver in the form of an amplifier. Advantage is taken of the natural resonance of the electrical circuit to provide for the deflection of the beam across the cathode ray tube and the "fly-back" of the beam to begin an additional or subsequent scan across the tube.
The principle advantages of these resonance scan systems are utilized in cathode ray tubes wherein a raster is provided on the tube face. In such applications wherein the required raster on the cathode ray tube is constant, as where the scan rate is held to a single value, such fly-back deflection systems are advantageously used.
The application of the present invention is in a cathode ray tube utilized as a display in a scanning electron microscope. In such a system, the scan rates may be varied for different magnifications to take advantage of other operating characteristics of the operating system (microscope) such as enhanced resolution at high magnification.
Alternative beam deflection systems known in the art are those which occur in vector writing systems. In such applications, the electron beam (of a cathode ray tube) is deflected from any one given position to another position by the driving of the appropriate current in the deflection coils. In these vector writing systems, the activity of the electron beam which is deflected is essentially nonrepetitive as the beam is caused to move from position to position on the face of the cathode ray tube. Thus, it should be realized that the advantages of the electronic symmetry of the resonance deflection systems cannot be advantageously used. Thus, these vector deflection systems require deflection amplifier drivers capable of generating requisite voltage to be applied to the yokes which control the beam so as to cause the beam to assume the requisite deflection according to the image desired on the screen. In these systems, it must be anticipated that the electron beam which is deflected may have to be driven from one side of the image device (CRT) to the other side. In most vector systems, the time available for the beam to be deflected is held to a minimum which requires, accordingly, the application of a large voltage on the deflecting inductor in order to accomplish the requisite deflection.
In any given deflection system, the deflection of the electron beam is accomplished by the electric field generated in the inductor. This field is effectively the product of the coil inductance and the current established within the inductor. Once these design parameters are established and the coil implemented into a system, field change is accomplished by a given voltage applied to the deflection coil, according to the well-known equation (1) below: EQU V=L (di)/(dt)
As is usual in electronic systems, the design parameters of the components are chosen on the basis of a "worse case" situation. Thus, the deflection coils are usually chosen so as to produce an electrical field sufficient to deflect the beam the furthest distance in the shortest anitcipated time.
In designing a deflection system for a CRT, the constraints for deflection power are usually the coil inductance and current rating for the yoke of the CRT. Once these parameters are established, the voltage to be placed on the coil is determined by the relationship of equation (1) above, namely, V=L (di)/(dt), where V is the voltage necessary across the coil, L is the inductance in henris and (di)/(dt) is the rate of change of current with respect to time.
In TV-type raster scanning, the (di)/(dt) parameter becomes important during retrace time when the current in the coil has to reverse direction and return to a starting value in a relatively short period of time. During a retrace, in the application of equation (1) (di)/(dt) is determined by the overall system requirements. As is recognized, the inductance L is fixed so the voltage V placed upon the coil has to comply in order to drive the current to the starting value.
FIG. 1 illustrates the typical current wave during a retrace and deflection period of a raster-type image on a CRT. In FIG. 1, t.sub.r represents the retrace time and t.sub.d represents the deflection time.
As the (di)/(dt) is different for the retrace time t.sub.r and the deflection t.sub.d so is the requirement for the voltage between these two times different.
As was indicated in conventional deflection systems, the voltage supply is made large enough to meet the worse condition, explicitly, the retrace time t.sub.r. The shortcoming of this approach is the power dissipation is unnecessarily high during the remaining portion of the circuit operation (t.sub.d). During this period of time, the voltage requirement of the yoke is considerably less as may be seen from the much lower value of (di)/(dt) from FIG. 1.
In the present invention, the voltage supply is conditioned to have two discrete values, one active for the retrace time t.sub.r and one active for the deflection t.sub.d. This approach minimizes the overall power requirements of the deflection system since, during retrace only will the output of the higher voltage source be operational. The motivation for using a deflection scheme such as our present invention in such usual CRT instruments as commercial television receivers is not evident. Since, as indicated, the requirements of such television systems may be satisfied readily with a resonance type deflection system, the single scan rate requirements of commercial TV receivers together with the relatively low resolution requirement of the spot of the beam do not require large power drives or particularly accurate voltage supply systems. However, in the case of applications of the present invention in such as scientific instruments, the reverse is the case.
The present invention finds application in the display monitors of scanning electron microscope (American Optical Coates & Welter Division's SEM's Model).
It is usual in such systems that high resolution monitors are employed including such as a 3,000-line CRT. It should be recognized that in the utilization of such high resolution monitors that the requirements on the deflection system are much more severe than those in the conventional TV systems. Spot size is required to be maintained at the design parameter throughout the deflection over the full raster and stability of the raster is required to be maintained to a high degree of accuracy. It is also common to synchronize the scan of the electron beam in a cathode ray tube to the scan of the beam within the scanning electron microscope to ensure presentation of the image upon the cathode ray tube. It should be recognized that the present circuit finds utility in this application since the high current for quick retrace in the deflection control is exercised only cyclically during retrace, reducing power loss in the retrace supply during this period.
One of the principal advantages of the present invention in such instrument applications is the lower power requirements over all of the deflection system. In scientific instruments high capacity voltage supplies are power consuming and thus heat-generators. As is recognized with precision scientific instruments, it is imperative that operating temperatures of instruments be maintained stable and in many cases heat sinking is required. These requirements are found in scanning electron microscopes. The inclusion of fans or other cooling devices in such delicate scientific instruments often interject noise and other unwanted detractors to the overall performance. A substantial reduction in heat generation occurs by the inclusion of the present invention. Thus, the provision of a "cool" deflection system greatly reducing heat sinking requirements within the instrument has produced energy savings in addition to improved operational characteristics. A collateral benefit in the instrument is the further avoidance of expensive high compliance, high voltage supplies.
These and other advantages of the present invention will be apparent from the subsequent description of a preferred embodiment of the invention.